A. Field of the Invention
The invention relates generally to reference voltage generators. More particularly, the invention relates to reference voltage generators that generate a conditions-insensitive reference voltage.
B. Description of the Related Art
Reference voltages are used in a wide variety of digital, analog, and mixed-signal circuits. Examples of circuits using reference voltages include analog-to-digital converters, digital-to-analog converters, and phase-locked loops. Analog-to-digital converters, for example, may use such reference voltages to accurately determine the magnitude of the analog input voltage—by comparing the input voltage to a well-known reference voltage—before converting the analog voltage to a digital signal.
The Brokaw cell is a popular reference voltage circuit that is widely used in many integrated circuits. The Brokaw cell requires a relatively high supply voltage, which limits the cell's application in today's advanced integrated circuits that operate with relatively low voltage power supplies. Many of today's advanced integrated circuits require relatively low voltage supplies to achieve lower power consumption and to generate less heat, among other things. Examples of devices requiring such low voltage supplies include mobile phones, personal data assistants, digital cameras, and other small, battery-operated devices. Alternative circuits to the Brokaw cell were developed that could operate with lower power supply voltages. While accomplishing lower supply voltage operation, however, these circuits generate reference voltages that include components that depend on the base currents of bipolar transistors used in the circuit.
For typical bipolar transistors having a large β value—the ratio of collector to base current—the additional term is small compared to other currents in the circuit. In many integrated circuits, reference voltage circuits are formed using parasitic bipolar transistors that have a relatively low β value. Thus, the base current term can affect the sensitivity of the reference voltage as the base current may vary with temperature, power supply, and load variations, for example.